Spark plug with an annular heat dam recess in the insulator nose



May 20, 1969 c; F. WRIGHT 3,445,710

SPARK PLUG WITH lAN ANNULAR HEAT DAM RECESS IN THE INSULATOR NOSE Filed Sept. 6. 1966 A /NvE/vroR GILBERT F. WRIGHT BY Iwkw? ATTORNEY United States Patent O U.S. Cl. 313-143 3 Claims ABSTRACT OF -THE DISCLOSURE A spark plug adapted to operate efficiently in hot and cold engines, that is either high compression or low compression engines, the spark plug is provided with a central electrode surrounded by a nose of insulation. The nose is surrounded by an annular recess and a skirt of insulation which functions as a heat darn. The skirt is positioned inside of the threaded spark plug body and is slightly spaced therefrom so as to shield this part of the body from the heat of the combustion gases.

This application is a continuation-in-part application of my copending application, Ser. No. 390,950, led Aug. 20, 1964, entitled Spark Plug, now abandoned.

This invention relates to spark plugs for use in internal combustion engines and in particular to a spark plug construction which will provide improved operating results throughout a wider range of engine operating conditions.

Modern internal combustion engines present a wide range of operating conditions. In particular, high compression engines operate at relatively high combustion temperatures Whereas low compression engines operate at relatively low combustion temperatures. In addition, the varying conditions under which a given engine operates can cause a substantial change in temperature of the combustion chamber, for example, mountain travel versus travel on level land, and cold weather operation versus hot weather operation. As a result of such different operating conditions, a variety of different types of spark plugs have been available, none of which provided optimum performance for the various operating conditions which may be encountered. In general, spark plugs are identified broadly as hot plugs and cold plugs which are respectively used with cold engines and hot engines.

Hot and cold plugs are so named because hot plugs are constructed to provide for less heat dissipation from the insulating nose cone around the center electrode to the water cooled engine block than is true of cold plugs and, therefore, run at a higher tip temperature than cold plugs for a given combustion chamber temperature. Since, for optimum operating and performance conditions, the tips of plugs should have approximately the same temperature, hot plugs are used with cold running or low performance engines and cold plugs are used with hot running or high performance engines. Of course, there is a Wide range between hot and cold engines which accounts for the fact that there are many different plugs available.

Generally speaking, there are two temperatures which must be taken into account in the construction of plugs, which are the non-fouling temperature and the preignition temperature of the insulator material disposed between the two electrodes. It has been found that, unless some annular portion of the exposed insulating surface of the ceramic nose cone has a temperature between ap- 3,445,710 Patented May 20, 1969 proximately 900 F. and 1300 F., the carbon and lead compounds formed adhere to the nose cone to cause fouling. Further, it has been found that, if any part of the electrode assembly, including the ceramic insulator, exceeds a temperature of approximately l700 F. with todays fuels, there will be pre-ignition. Accordingly, to prevent fouling, some annular portion of the exposed insulator surface must have a minimum temperature of at least 900 F. and to prevent pre-ignition, no portion of the exposed insulator surface (or electrodes) may have a temperature which exceeds l700 F. The specified non-foul ing and pre-ignition temperatures vary somewhat with the particular insulating material and the particular fuel used, and, therefore, can only be regarded as typical values. Further, there is some dispute whether the fouling below the pre-ignition temperature is due solely to carbon deposits as contrasted with lead deposits from the fuel. Some authorities believe that lead deposits cause high temperature fouling at temperatures usually higher than the pre-ignition temperature.

The problem encountered with prior art plugs is the narrow engine operating range over which they meet the above two temperature requirements. For example, a hot (long nose) plug designed for use with low grade fuels or with a cold engine has a tendency to cause pre-ignition when higher grade fuels are used, or when severe demands are made on the engine, particularly in hot weather because a portion thereof reaches the pre-ignition temperature under such conditions. Conversely, a cold (short nose) plug designed for use with a high grade fuel 0r with a hot engine has a tendency to foul when lower grade fuels are used, or when light demands are made on the engine particularly in cold weather or when starting, because no annular portion of the exposed ceramic insulator surface has reached the minimum nonfouling temperature of about 900 F It is, therefore, an object of the present invention to provide a spark plug of improved construction which will operate properly throughout a substantially wider range of combustion chamber temperatures than has been possible heretofore.

It is another object of the present invention to provide an improved spark plug construction in which the metal electrode housing or shell operates at lower temperatures than conventional plugs to increase the heat dissipation from the insulator to the engine block.

It is a further object of the present invention to provide an improved spark plug construction which is nonfouling at the lower combustion chamber temperatures and which is non-preigniting at the higher combustion chamber temperatures.

`It is still another object of the present invention to provide an improved spark plug construction in which the ceramic insulator is shaped and spaced to prevent fouling at lolw combustion chamber temperatures and pre-ignition at high combustion chamber temperatures, those means being a shorter nose than is usually employed with comparative hot plugs and a longer nose than is usually employed with comparative cold plugs, and a skirt shielding the outer housing which makes the plug hot for cold operation and cold for hot operation.

Briefly, the spark plug of the present invnetion accomplishes the above-stated objects by utilizing a uniquely shaped insulator body which has a nose or sleeve portion surrounding the center electrode of the spark plug, and a skirt or shield portion immediately adjacent to and spaced from the inside surface of the body or shell of the spark plugpThe nose portion and the skirt portion form the sides of an annular groove having a selected critical depth and minimum Width for optimum performance as will become better understood in the ensuing detailed description.

The nose portion of the spark plug of this invention resembles that of a fairly cold plug and, therefore, allows for the rapid heat dissipation usually associated with cold plugs. The skirt portion is not in contact with the plug body, but is spaced therefrom for better thermal insulation. As a result thereof of such spacing, the end of the skirt portion has a tendency to warm up rapidly and to reach the minimum non-fouling temperature for cold engine operation. Accordingly, the spaced skirt portion in combination with the annular groove is believed to give the plug its hot plug characteristics. Further, the skirt portion protects the plug body from the heat of the combustion chamber so that the plug body remains much cooler than Iwith conventional plugs which facilitates heat dissipation from the ceramic insulator to the engine block. This feature, together with the relatively short nose portion, is believed to give the plug its cold plug characteristic.

The various objects and features of advantage will become more apparent from the following detailed description rwherein reference is made to the accompanying drawings in which:

FIGURE 1 shows in solid lines a preferred spark plug construction for achieving the objects of the invention. The spark plug is shown in operational position in an engine block which is represented in dot-dash phantom outline. 'Ihe upper end of the spark plug can be of conventional construction and is therefore shown in elevation whereas the novel lower end of the plug is shown in cross section taken along the axis of the plug; and

FIGURE 2 is an enlarged view, partly in cross section, of the tip portion of the spark plug shown in FIGURE l.

Referring now in detail to the drawing, there is shown the improved spark plug threaded in operating position in an engine block 12. Spark plug 10 comprises a metal body or shell 14, an insulating core 16 disposed in body 14, a metallic central pin 18 disposed in core 16 and a metallic tab 20 welded to body 12. The electrodes for the spark plug are formed, in the conventional manner, by central pin 18 in core 16 and tab 20.

Body 14 may be of conventional construction and is substantially so shovwn in the drawing. More specilcally, body 14 is of elongated hollow construction and has a small diameter inner end portion 22 provided with threads 24. Threads 24 are, of course, shaped to engage matching threads on engine block 12. Body 14 also has a large diameter outer end portion 26 which forms a shoulder 28 at the end of threads 24 which engages a sealing gasket 29 for sealing engagement with engine block 12. In order to facilitate gripping plug 10 with a wrench, body l14 is provided with a at-sided section 30, preferably hexagonal. To securely hold core 16 within body 14 and provide a seal therewith, the outer end of body 14 is provided with a swedged-over lip 32. A metallic sealing ring 34 is conventionally positioned for compression between core 16 and lip 32.

Core 16 can be made of any insulating material conventionally employed for the cores of spark plugs, such as, for example, aluminum oxide ceramic. However, the shape of the inner end of core 16 is different from that employed for conventional spark plugs. Core 16 has a small diameter or nose portion 36 at its inner end and directly surrounding center electrode pin 18. The inner end of core 16 also has a cylindrical or skirt portion 38 which is dimensioned so that it does not touch the inner surface of inner end portion 22. As shown in the drawing, nose portion 36 and skirt portion 38 form between them an annular groove 40. Core 16 is also provided -with a large diameter portion 42 which forms a shoulder 44 for engaging a metallic gasket 46 which, in turn, engages a cooperating shoulder 48 on the inside of body 14.

In conventional spark plugs, there is no skirt portion 38, and nose portion 36 normally tapers upwardly and outwardly to flare into shoulder 44. The length of the conventional conical nose portion determines the hotness or coldness of the plug. As a result of such conventional construction, the inside surface of the inner portion 22 of body 14 is directly exposed to the combustion gases, from the tip of inner end 22 all the way to shoulder 48.

The heat path for carrying heat from the nose portion of a conventional spark plug is up along the nose portion, across a shoulder such as 44, across a gasket such as 46, across a shoulder such as 48, through body 14, and across a sealing gasket such as 29 to the engine block which is cooled by water or air and forms the necessary heat sink. A relatively small amount of the heat from the nose portion might also pass across the upper part of threads such as 24. Since the conventional spark plugs must dissipate not only the heat from the nose portion but also the extreme heat from the exposed inner end of the spark plug, the gasket such as 29I runs extremely hot, say, in the neighborhood of 500 F. when the nose is at the proper temperature of between 900 F. and 1300io F. When such a conventional spark plug is then exposed to higher combustion temperatures, the heat paths across the body gasket and the body threads are already substantially loaded and, in effect, form heat dams resulting in poor heat dissipation which causes the nose portion temperature to go quickly to the pre-ignition temperature.

-In contrast with conventional spark plugs, the spark plug shown in the drawings permits gasket 29 to run at much lower temperature. The reason for the lower ternperature is that skirt portion 38 shields the inner surface of inner end 22 from the combustion gases. Of course, skirt portion 38 will itself be exposed to the combustion gases, but the thermal conductivity of the insulating material of skirt portion 38 is very poor relative to that of body 12 so that substantially less heat will be passed through gasket 29.

- In addition, the surface area of nose portion 36 plus skirt portion 38 exposed to combustion gases need not be greater in the construction shown in the drawing than the area of the nose portion alone in a conventional plug. The reason, therefore, is that the length of nose portion 36 is considerably shorter than in a conventional plug, as will hereinafter be explained in greater detail, and still obtains substantially the same length of exposed insulating path. In other words, the exposed insulating path of conventional long nose spark plugs which extends normally to the shoulder, such as 44, may be regarded as folded over in the present invention to give the re-entrant surface path shown in the drawing between center electrode 18 and body 14. Thus, the improved construction makes it possible to shield the inner end 22 of body 14 from the combustion gases without substituting an equal surface area of additional insulating material.

In order to realize the fullest possible benefits from the improved spark plug structure of this invention, it iS necessary that skirt portion 38 be spaced apart from inner end 22 for maximum thermal insulation therewith in order to maintain inner end 22 as cool as possible, and thereby confine the heat transfer from the spark plug to engine block 12 through the intended heat path across gasket 29. The spaced apart distance, indicated by dimension B, should, however, be suiciently small to prevent any appreciable flow of gases between the skirt portion and the inner end of the spark plug body to avoid heating inner end 22 and, therefore, gasket 29. It has been found that these objects are accomplished by making dimension B anywhere between l mil and 10 mils (mil=onethousandths of an inch).

The depth of annular groove 40, indicated by dimension A, should also be maintained between certain limits. This dimension corresponds somewhat to the nose portion length of conventional plugs which have a typical length of 5A for standard plugs, 1" for exceptionally hot plugs, and 14a for exceptionally cold (racing) plugs. For the improved plug construction of this invention, dimension A is selected to be approximately M32" `with limits of i/s. More particularly, for standard engine operations, dimension A is selected as 7/32", for operations requiring extremely cold plugs dimension A may be decreased to ygg", and for operations requiring extremely hot plugs dimension A may be increased to 9/32.

The width of annular groove 40, indicated by dimension D, is also important and should be maintained within certain limits for optimum operation. It has been found that for a 14 millimeter spark plug, dimension C should not be less than about 40 mils and not more than about 70 mils, a good average width being 50 mils. If the width of annular groove 40 is much less than 40 mils, there is insufficient space for the combustion gas to enter and to heat the insulator to the required minimum nonfouling temperature. If the width is much greater than 70 mils, the thickness of skirt portion 38 (dimension E) and of nose portion 36 (dimension C) becomes too thin to provide the desired mechanical strength. A good rule of thumb to follow is to make dimensions C, D and E substantially equal to one another. Since the diameter of pin electrode 18 is usually in the neighborhood of onequarter of the inside diameter of inner end 22, the dimensions C, D and E may all be substantially equal to 1A of the outside diameter of skirt portion 38 (which is substantially equal to the internal diameter of inner end 22).

A spark plug constructed as shown in the drawing and described in the specification has been found to operate properly at relatively low combustion temperatures and also at relatively high combustion temperatures, and therefore, has a much wider operating range than conventional spark plugs. The reason for the increased operating range is not entirely understood, but is believed to include the following considerations. During cold engine operation, normally requiring a hot (long nose) plug, the tips of nose portion 36 and skirt portion 3S are directly exposed to the combustion gases and will, therefore, quickly reach the required minimum fouling temperature. The heating process is aided by the close proximity between nose portion 36 and skirt portion 38, and there is little heat loss because nose portion 36 is protected by the heat dam formed by skirt portion 38.

During hot engine operation, normally requiring a cold (short nose) plug, body 14 and, therefore, gasket 29 remains relatively cool resulting in a higher than normal temperature differential across gasket 46. This higher than normal temperature differential facilitates heat conduction from core 16 across gasket 29 to engine block 12. This effect is most pronounced when core 16 becomes very hot since the rate of heat conduction is proportional to the temperature gradient across gasket 46, and the gasket temperature is substantially constant. As long as core 16 is relatively cool, the temperature differential is relatively small which is believed to explain the relatively smaller transfer of heat during cold engine operation. However, with conventional spark plugs, the spark plug sealing gasket runs at a very much higher temperature because of the direct exposure of the inner end of the body portion to the combustion gases and, therefore, the heat transfer is more ineflicient.

While the above detailed description has shown, described and pointed out the fundamental novel features of the invention as applied to various embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated may be made by those skilled in the art, without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. In a spark plug for internal combustion engines, said plug being adapted for eicient operation in engines having relatively low combustion temperatures as well as engines having relatively high combustion temperatures, the combination of an elongated hollow body of heat conductive material having an inner end adapted to be threaded into the block of the engine, an elongated insulator core received within said body, said insulator core having a skirt portion adapted to be positioned in the threaded portion of said hollow body, the outer surface of said skirt portion being spaced from the inner surface of said threaded portions of said body by an annular gap which forms a thermal barrier between said skirt portion and said inner surface of said body, said skirt shielding said threaded portion of said body from excessive heating by the combustion gases, said insulator core having a centr-al portion thereof in thermally conductive conta'ct with said body above said threaded portion so that heat received by said insulator core is conducted therethrough to said body, said insulator core having a nose portion positioned in said skirt portion and spaced therefrom by an annular recess, a cent-ral electrode mounted in and extending through a centrally disposed bore in said insulator core, said electrode projecting from the inner end of the nose portion of said core, another electrode attached to the inner end of said body and projecting radially inward over the inner end of said central electrode to provide a spark gap therebetween, said annular recess exposing sunfaoes of said nose to the combustion gases and said skirt providing a heat darn around said annular recess to reduce escape of heat to said body so that said nose is heated to the minimum temperature necessary to prevent Ifouling thereof.

2. In a spark plug ffor internal combustion engines, said plug being adapted for eicient operation in engines having relatively low combustion temperatures as well as engines having rela-tively high combustion temperatures, the combination `as set forth in claim 1 yfurther characterized in that the wall thickness of said skirt portion and said nose portion are substantially equal to one another.

3. `In a spark plug for internal combustion engines, said plug being adapted for efficient operation in engines having relatively low combustion temperatures as well as engines having relatively high combustion temperatures, the combination Ias set forth in claim 2 further characterized in that the facing Walls of said annular recess are substantially equally spaced from each other at different altitudes of said recess.

References Cited UNITED STATES PATENTS 1,341,747 6/ 1920 Ingels 313-143 X 1,352,149 9/1920 Schmidt.

2,208,178 7/ 1940 Berstler 313-143 2,353,620 7/1944 Weinerth 313-143 X JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

U.S. C1. XJR. 313-132 

